Uqayl narrates from Ibn Shihab that he was asked about a man who had committed zina with his wife's mother, and he said that Sayyiduna Ali (may Allah be pleased with him) said: "The unlawful does not make anything of the lawful unlawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13963